1. Field of the Invention
This invention generally relates to a surface acoustic wave device that employs a piezoelectric material, and more particularly, to a surface acoustic wave device having multiple interdigital transducers (hereinafter simply referred to as IDT) on a piezoelectric material substrate (hereinafter simply referred to as piezoelectric substrate).
2. Description of the Related Art
In these years, the above-mentioned type of filter, which is composed of surface acoustic wave (hereinafter referred to as SAW) device having multiple IDTs on the piezoelectric substrate, has been employed for a bandpass filter in a television set having a frequency range of 30 MHz to 400 MHz and an RF filter in a mobile telephone having a frequency range of 800 MHz to several GHz. An IDT includes a pair of comb-like electrodes. Each comb-like electrode is composed of a bus bar and electrode fingers having first edges connected to the bus bar and second edges that are open. A pair of comb-like electrodes is arranged so that the electrode fingers of the comb-like electrodes are alternately crossed or interleaved at regular intervals. In other words, the interleaved electrode fingers are alternately connected to two bus bars. A SAW is generated by applying an alternating voltage across the pair of comb-like electrodes. The SAW has a frequency response by which a filter having a desired frequency characteristic is obtainable.
FIG. 1 shows a filter with the SAW. Japanese Patent Application Publication No. 10-41778 (hereinafter referred to as Document 1) discloses this type of filter. Referring to FIG. 1, there are arranged a first IDT 10, a ground electrode 20, and a second IDT 30 on a piezoelectric substrate 1. The first IDT 10, the ground electrode 20, and the second IDT 30 are adjacently arranged in a direction of the SAW propagation. The ground electrode 20 is arranged between the first IDT 10 and the second IDT 30, serving as a shield electrode. The first IDT 10 serves as an input electrode (or output electrode) and the second IDT 30 serves as an output electrode (or input electrode). The ground electrode 20 prevents electromagnetic coupling of the IDT 10 and the IDT 30. Also, the ground electrode 20 is arranged on a tilt in order to prevent the SAW that travels from the IDT 10 (or the IDT 30) from being reflected by the ground electrode 20 and returning to the IDT 10 (or the IDT 30).
The IDT 10 includes a pair of comb-like electrode 10a and 10b. The comb-like electrode 10a includes a bus bar 12a and multiple electrode fingers 14a. The comb-like electrode 10b also includes a bus bar 12b and multiple electrode fingers 14b. The open edges of the electrode fingers 14a face those of the electrode fingers 14b, which are referred to as overlapping parts or crossing portions. The overlapping parts of the interleaved electrode fingers that face each other are involved in excitation of SAW. As shown in FIG. 1A, an electrode finger pattern is weighted. The electrode finger pattern is defined as a pattern formed by the electrode fingers. The electrode finger pattern may be weighted by, for example, apodization. By this apodization, lengths of the electrode fingers in the overlapping parts (hereinafter referred to as aperture length) vary in the propagation direction. The aperture lengths are relatively small in the vicinity of both sides of the IDT 10, which is defined as small overlapping parts. On the other hand, the aperture lengths are relatively large around the center of the IDT 10. The aperture length is proportional to excitation intensity. Therefore, the strong SAWs are generated around the center of the IDT 10, and weak SAWs are generated in the vicinity of both ends of the IDT 10. The frequency characteristic may be altered by changing the weight by apodization.
FIG. 1B shows an electrode finger pattern that is weighted by apodization. An apodized electrode or IDT denotes an electrode or IDT having an electrode finger pattern that is weighted by apodization. Referring to FIG. 1B, there is provided an electrode finger pattern having relatively long overlapping lengths (aperture lengths) of the electrode fingers around the center of the IDT 10 (which is referred to as main lobe). On both sides of the main lobe, there are provided multiple electrode finger patterns having the aperture lengths that are shorter than those of the main lobe (which is referred to as side lobe). The aperture lengths become shorter as the electrode fingers get closer to the ends of the IDT 10. Referring to FIG. 2, a main lobe 21 includes three lobes. Two second lobes 22 are provided on both sides of the main lobe 21. Small electrode finger overlapping parts 23 are provided between the three main lobes 21 and between the main lobe 21 and the second lobe 22. Generally, the small electrode finger overlapping parts 23 occupy 50 to 60 percent of the entire electrode finger pattern. It is general in design to have a sufficient impulse response time and a high-order side lobe in order to gain a favorable filter characteristic of the SAW device.
Referring back to FIG. 1A, the IDT 30 also includes a pair of comb-like electrodes. However, the IDT 30 is not weighted, which is different from the IDT 10. That is, all the electrode fingers have an identical overlapping length. This electrode pattern is referred to as a normalized electrode pattern, and the IDT having the aforementioned pattern is referred to as a normal IDT.
The bus bar 12a of the IDT 10 is connected to an electrode pad 15. The bus bar 12b is connected to an electrode pad 16. The bus bars of the IDT 30 are respectively connected to electrode pads 17 and 18.
A filter with the above-mentioned configuration serves as a bandpass filter.
However, there is the problem in that the above-mentioned conventional SAW device does not have a sufficiently large stopband attenuation.